The present invention relates to a fixing device for heat-fixing an unfixed toner image (yet to be fixed toner image) on a sheet (paper) which is a recorded medium equipped in an image forming device such as a copying machine, laser beam printer, facsimile and the like using an electrophotograph method.
The image forming device utilizing an electrophotograph method creates a manifest image by powder toner from a latent image formed on a photosensitive body having a photosensitive layer which works as a recording medium, and transfers said manifested toner image onto a sheet-type paper which is the recorded medium, and since said toner is yet to be fixed, heat is applied to melt the toner, and then pressure is added thereto to fix said toner image onto the paper. In order to do so, a fixing device is provided on the downstream side of a paper conveyance passage which passes through an image forming region, for example, just before a discharge portion of the paper.
One example of conventional fixing device disclosed for example in Japanese Laid-Open Patent Publication No.6-202518, is formed of a heat roll having in the interior thereof a heater lamp comprising a halogen lamp as the heating source and a pressurization roll pressurized to said heat roll by a predetermined pressurization force, and is positioned on the upstream side of a discharge portion of the conveyance passage which passes through an image forming portion, wherein a toner image on a sheet-type paper formed at the image forming portion is contacted to the heat roll, and by the heat of the heat roll and the pressurization force, the toner image is heat-fixed to the sheet.
In the fixing device, the heat roll contacts the toner when fixing the toner on the sheet of paper, so the toner adheres to the heat roll. In order to remove the toner on the heat roll, a cleaning web and the like are mounted. In this case, so as to reduce the cleaning load, an application device for applying a release agent of a silicon oil and the like on the surface of heat roll is equipped, thereby performing the fixing operation while aggressively preventing toner from being adhered to the heat roll.
The application device supplies and applies the release agent stored in a tank by sucking up the release agent with an application member to an application roll being pressed against and rotated against a surface of the heat roll, thereby applying the release agent applied to the application roll onto the heat roll. The application member has one end portion being submerged to the release agent in the tank, and the other end being pressed against the application roll. The application member sucks up the release agent utilizing a capillary phenomenon (action), and applies the same to the application roll on the pressed portion. Thereby, the application member comprises of a felt and the like.
The application of release agent by the application member comprising a felt and the like is not stable of its application quantity. Therefore, the application quantity to the application roll is uneven. So when applying the release agent to the heat roll, an excessive application and the like will occur, which causes fouling of the sheet by the release agent or the fouling of the interior of the image forming device including the fixing device by the dripping of the excessive release agent. Therefore, a blade for removing the excessive release agent applied by the application member and for limiting the amount of the release agent on the surface of application roll so as to form an even layer (film thickness) is mounted on the downstream side of the rotational direction of the application roll.
The blade is a rubber blade for example having elasticity, wherein the tip end portion thereof is pressed against the surface of application roll. Therefore, a uniformly controlled (limited) release agent layer (film) will be formed on the surface of application roll, even in the direction of rotational axis, after the blade has passed through the surface.
The excessive release agent removed from the surface of application roll by the blade will either be sucked by the application member and returned to the tank, or gets over the application member and returned to the tank. Thereby, the unnecessary release agent will be collected and circulated for supply.
According to the conventional fixing device disclosed in the above-mentioned publication, the release agent could be uniformly applied to the heat roll, which prevents the excessive application of the release agent to the roll. Therefore, fouling of the paper by the release agent will not happen.
However, when removing the excessive release agent by the blade, the excessive release agent will, get over the application member to be collected by a tank and to be reused, but on the other hand, a part of the excessive release agent will drip off from the both end portions of the blade, which will not be collected by the tank, fouling the interior of the fixing device with the release agent.
This happens because the blade and the application member are mounted with a sufficient space in between so as to maintain the excessive release agent. When the amount of release agent being collected at this area increases, the excessive release agent will drip off from the both end portions of the blade.
The above-mentioned problem will now be explained in detail. By the starting of operation of the fixing device, the application roll will be rotated, and the release agent being sucked up by the capillary action of the application member will be supplied to the rotating application roll by the viscosity of the release agent. The application member supplies the release agent to the application roll, and at its tip portion, the release agent will be in an unsaturated state. So as to compensate the lack of release agent at the tip portion, the release agent will further be carried up by the capillary action, and will be supplied to the application roll. When such supply of the release agent is continued, the adherence quantity (supply quantity) of the release agent from the application member to the surface of application roll will, though the amount may differ by the characteristics and the surface roughness of the material of application roll, be approximately in the order of 10.sup.-3 g/cm.sup.2.
However, the amount of release agent necessary for fixing the image, or the adherence quantity of the release agent after the passing of the blade, is approximately 1/10 of that amount. Therefore, most of the release agent being applied by the application member is wiped away by the blade. The excessive or surplus release agent generated as above will travel along the edge portion which is the contacting portion of the application roll and the blade, and will drip off from the both side end portions of the blade as release agent drops. The interior of the fixing device and the image forming device will be fouled by the release agent if the dripping release agent will not be collected.
Therefore, the tank must be large enough to cover the whole lower area of the application roll for collecting the excessive release agent, and the tank should be positioned so as to protrude from the both side end portions of the application roll in the direction of rotational axis. Therefore, the application device itself will become large, and as a result, the whole fixing device will become large. Further, there is a need to mount a means for collecting the release agent, which will raise the cost.
Therefore, a device is considered where the amount of release agent supplied to the application member for applying the release agent to the application roll is limited, so that no excessive release agent will be applied to the application roll. For example, the above-mentioned Japanese Laid-Open Patent Publication No.6-202518 discloses a plan to stabilize the supply quantity of the release agent by mounting a filter cover made of a porous fluoroplastic and the like to the contacting portion of the tip of the application member and the application roll.
However, it is very troublesome to mount a fluoroplastic cover to the application member itself, and if the porous state is uneven, the quantity of supply will be changed greatly, and uniform application may not be performed by such supply disorder. Further, the application member will become expensive.
Further, the above-mentioned publication also proposes to remove the application member, and to mount a supply pump of the release agent, which pumps up the release agent and to spray the release agent through an application nozzle to the application roll. According to such structure, the amount of release agent to be supplied by the supply pump could easily be adjusted, which enables to control the supply of excessive release agent, and to reduce the amount of release agent to be removed by the blade, thereby enabling to restrict the dripping off and the like of the release agent.
However, such device requires a large-scale means such as supply pumps and application nozzles. This not only increases the size of the device, but also raises the cost greatly, so it is not suitable for the miniaturization or the cost-reduction in a device.
On the other hand, in the case where the excessive release agent is removed by the blade and collected by the tank for the reuse, the release agent may be deteriorated by the long-term use. In recent years, though, the release agent to be used in such devices utilize a silicon system oil which is chemically highly stable, and the problem of deterioration is solved to some extent.
However, by recycling and reusing the release agent, foreign matters such as the additive included in the toner or the toner itself will simultaneously be collected with the collection of the release agent. That is, the foreign matter adhered to the heat roll performing the fixing process will be adhered to the application roll, and will manage to pass through the application member and removed by the blade, and collected to the tank. As a result, many impurities will be mixed into the release agent, damaging the release ability or the glossiness of the image, and creates the cause of offsets.
This problem could not be solved by the above-mentioned methods, and it is a problem that could not be solved when recycling and reusing the release agent.
Therefore, a supply pump could be mounted without reusing the release agent, and by limiting the amount of supply, the release agent corresponding to the reduced amount of release agent could be applied. However, in such case, the size of the device will be increased as was explained above, and the problem of raise in the cost is left unsolved.